calc 2.13.0.1 release

The following are the changes in this release:

Replaced /usr/local with the use of ${PREFIX} in calc Makefiles.

The ${PREFIX} is not the same as ${T}.  The ${T} specifies
a top level directory under which calc installs things.
While usually ${T} is empty, it can be specific path
as if calc where "chrooted" during an install.
The ${PREFIX} value, during install, is a path between
the top level ${T} install directory and the object
such as an include file.

Corrected a few more typos in Makefile comments.

Added Makefile.local, a file with a single comment.  The main
Makefile includes Makefile.local just before the first all rule.
One may override any Makefile setting by modifying Makefile.local.
For example, Makefile.local could force BLD_TYPE:

HAVE_STRING_H:= YES
HAVE_TIMES_H:= YES
SED:= /usr/local/bin/nsed

Added ${LOC_MKF} to specify the make of the file that is
included just before the all file.  So one could place
the above override lines into a different file and call
make changing the ${LOC_MKF} value.  For example:

make LOC_MKF=Makefile.private clobber all chk

Updated HOWTO.INSTALL to mention Makefile.local.